Thursday, 14 April 2011

Crazy ideas... PV/T ... heat pumps

At the end of the day, in the final design, the roof is simply a
photovoltaic solar array, but many ideas preceded this. If you look at
efficiency, electric solar panels are not terrible good at converting
the sun's bounty into usable means, changing less than one fifth of
solar energy into electricity. Thermal panels--turning sunlight into
heat, either directly using water or using a coolant--are much more
efficient, getting up to half of the sun's energy into hot water.


There are also panels that produce both heat and electricity. Solar
panels become less efficient as they get hotter, dropping up to one
percent per degree centigrade they get hotter, so running a coolant
through the panels can make them produce more electricity, as well as
producing heat, so rather than splitting heat and electricity, hybrid
photovoltaic thermal (PV/T) panels give you both. One problem seems
to be that if the temperature is high enough to generate hot water,
electric power will reduce 10-20%.

We looked at some panels from a Turkish company called Solimpeks,
although the cost was very high. Japanese-made panels are all
registered and qualify for government subsidies for installation and
purchasing electricity, but these are not registered and do not
qualify. A Japanese manufacturer did produce some PV/T panels a few
years ago, but they did not take off. In fact Japan falls well short
of its potential in solar thermal systems.

Hot air

When roof-top photovoltaic panels are installed, they usually have an
air channel running behind them, which is important for cooling.
Another idea was taking heat off the air flowing behind the panels
using an atmospheric heat pump. To make the heat pump more effective,
this should be done at the top of the roof, taking heat off the air
coming out. However, if the air was cooled on the way into the channel
under the panels, it would reduce the temperature on the panels and
increase their efficiency. An advantage of this system would be that
the heat pumps could be used from grid electricity on cold days with
no sunlight (if it snows all day, for example), so no backup heating
system is necessary.

Heat pumps are becoming popular in Japan for generating hot water (the
system is know as Eco cute, "cute" sounding like "kyuto", the Japanese
for "boiler"). Usually, they switch on at night to use off-peak grid
power. At night time, especially in winter, the air is at its coldest,
so it is the least efficient time for running the heat pumps, although
is the cheapest time for using electricity so the "eco" may be more to do with economy than ecology. Heat pumps have an average
COP (Coefficient of Performance) of around 3, so you need to spend 1kW of electricity to generate
3kW of heat. As the temperature drops, the COP drops until the heat
pump is doing no better than an electric heat element--the kind you
get in an electric kettle--at which point the system can switch over
to an electric heat element.

Getting three kW of heat for every kW of electricity may sound good,
but when you consider where the electricity is coming from, both in
terms of the original fossil fuel, and the distance it travels, one kW
of electricity actually takes almost three kW of fossil fuel to make
(the figure the Passive House Institute uses is 2.7) so it's not much
better than using fossil fuels directly. If you're using the
electricity from a solar panel to drive a heat pump, it will be less
efficient than a solar thermal panel.

A fairly popular solar heating system in Japan is known as OM solar, which takes heat through the
roof, collects it at the top and then pumps it down for under-floor
heating in the winter, or passes it through a heat-converter to heat
water in the Summer. This system needs a backup heat source, which is
used as the primary water heater in the winter.

If solar panels were placed at the top of an OM solar roof (as they
show on their website), optimisation of the two systems would be
working against each other: the heat system wanting the panels as hot
as possible, and the power system wanting them as cool as possible. OM
solar would work much better with solar panels on the lower part of
the roof.